In 2010 the United States Census Bureau will once again conduct the decennial census. But apart from the interesting bits of information we will discover, such as the average number of bathrooms in an American home, why does the Bureau undertake this daunting task every ten years?

It’s because the United States Constitution requires it. In Article One, Section Two, the original text reads as follows:

Representatives and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers, which shall be determined by adding to the whole Number of free Persons, including those bound to Service for a Term of Years, and excluding Indians not taxed, three fifths of all other Persons. The actual Enumeration shall be made within three Years after the first Meeting of the Congress of the United States, and within every subsequent Term of ten Years, in such Manner as they shall by Law direct. The Number of Representatives shall not exceed one for every thirty Thousand, but each State shall have at Least one Representative....

How we count the persons has thankfully changed since 1787. But the mandate of an “actual enumeration” remains. In fact, in the 1990s, the Census Bureau announced a plan to use statistical sampling to help achieve the required enumeration, rather than an actual headcount. After making its way through the courts, the Supreme Court ultimately ruled that sampling may not be used (see http://supct.law.cornell.edu/supct/html/98-404.ZO.html). But that is a topic for another day.

The Constitution requires an enumeration every ten years, as we read above, for the purpose of apportioning Representatives in the House of Representatives “according to their respective numbers.” Other than the specifications that each state must have a Representative, and that the number of Representatives shall not exceed one for every thirty thousand, the Constitution provides no guidelines as to how the Representatives shall be apportioned. This is where the fun begins.